There are many applications in ophthalmological research and treatment which require directing a beam of energy--such as a laser beam or an image projecting liquid beam--to a selected location on or in the eye of a subject or patient. In such applications the need often is to maintain the directed beam in the specific selected location regardless of eye movements (voluntary or involuntary) on the part of the subject. Prior to this invention, no practical means of providing such fixation was known.
The need for such a stabilized visual system is particularly acute in the eye treatment known as photocoagulation (both with coherent--laser--and incoherent light). There are a number of coagulator procedures for which the clinician would like to eliminate the effects of a patient's eye movements during treatment. This important application of the invention is one having most stringent requirements; therefore, the invention is described in connection with photocoagulation (specifically laser photocoagulation). It is to be understood, however, that the invention is broadly applicable to any situation requiring that a beam be fixed (stay) at a specific location in the eye regardless of eye movements.
Laser photocoagulation is a recognized technique of modern ophthalmological practice. With this treatment, "burn" lesions are created at the fundus by means of the focused energy of a laser beam. Currently available coagulators are generally built in the style of a binocular, slit lamp apparatus with facility added for a steerable laser input. In order to direct the laser beam to the desired fundus location, means are provided for illuminating the fundus and for energizing the laser beam at low power. When the operator is satisfied with the alignment, he energizes the full power of the beam, for example, by means of a foot switch. Separate controls are available to preset the intensity and duration of each flash. An example of such a system is described and claimed in U.S. Pat. No. 3,703,176, issued Nov. 21, 1972 in the names of Arthur Vassiliadis, Harold C. Zweng, Norman A. Peppers and Lloyd E. Alterton and assigned to Stanford Research Institute, now known as SRI International, and Stanford University.
Most current photocoagulation techniques depend on the use of a large contact lens placed on the eye of the patient. Although it may be possible to operate the eye tracker satisfactorily in the presence of such a contact lens, a preferred method would not require a contact lens to be used.
The fundus illumination and monitoring system shown here, which includes means for entering a steerable laser beam, was developed by the Eye Research Institute of Boston. The main feature of this photocoagulation method is that it does not require the use of a contact lens on the eye of the patient. A primary motivation in developing this technique was to improve the optical quality of the photocoagulation system, inasmuch as the contact lens causes large variations in the shape of the laser beam reaching different locations of the fundus. The fact that this method of photocoagulation does not require a contact lens makes it preferable in a stabilized system of the type described here, although many variations are also possible.
Ophthalmologists disagree on whether a laser should be used on the eye. There is no question that the laser provides useful treatment. Some ophthalmologists, however, are of the opinion that the chance of damage to the eye is too great, particularly since the operator cannot follow sudden patient eye movements fast enough to be sure the beam will hit the desired area. Even the ophthalmologists who regularly use laser photocoagulation do not use the treatment under certain circumstances where it might otherwise be beneficial. For example, unless the situation is very serious, an operator will generally avoid using the laser coagulator too close to the patient's fovea, because a sudden eye movement could bring the beam onto the fovea and cause serious damage to the patient's vision. Sudden eye movements at times occur reflexively in response to the impinging high power laser beam. Therefore, in many procedures using the laser photocoagulator, fear of eye movement dictates using very short, high energy bursts of the laser beam when longer applications at a lower energy level might be more beneficial. A system which allows the laser beam pattern to be directed to a specific location within a patient's eye and maintained at that location even though the eye moves will eliminate such concerns. In some cases where the retina lifts from the choroid and fills with fluid, the treatment consists of 100% beam coverage of the affected area. If the effects of the patient's eye movements are eliminated, the instrument can be programmed, like an automatic sewing machine, to cover the area in any prescribed pattern. Or, in cases of widespread retinal detachment, where thousands of beam applications, or "stitches", are required, the procedure can be lengthy. With the patient's eye movements under control, it is possible to automate the procedure, again like an automatic sewing machine.
Getting patient's eye movements "under control" means either eliminating them by drugs, a procedure that has its own dangers and is generally avoided in clinical practice, or measuring the patient's eye movements and moving the beam so as to compensate automatically for the eye movements. The present invention takes the second approach and, as far as is known at the present time, the art does not contain other attempts to accomplish the same result using this approach.